(a) Field of the Invention
The present invention relates to a method for preparing nickel fine powder and more specifically to a method for preparing nickel fine powder, which is principally suitable for use as a material for an internal electrode of laminated ceramic condensers, whose particle size distribution is sharp and which has a low degree of agglomeration and a paste containing the nickel fine powder is excellent in filling properties.
(b) Description of the Prior Art
The laminated ceramic condenser is a condenser produced by alternately putting ceramic dielectric materials and internal electrodes into layers, followed by bonding these layers under press and firing the resulting assembly to thus unite the layers with each other. On the other hand, techniques have been developed and advanced, in which a base metal such as Ni is used instead of noble metals such as Pt and Pd conventionally used as materials for such internal electrodes.
There have also been proposed a variety of methods for preparing the material, i.e., nickel powder along with the development and/or advancement of such techniques. A typical method for preparing the same includes a dry method such as a gas phase reduction of nickel chloride vapor with hydrogen as disclosed in Japanese Un-Examined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Hei 8-246001, but the wet method which comprises reducing a nickel ion-containing aqueous solution with a reducing agent under specific conditions to thus separate out nickel has many advantages including economical one from the viewpoint of the energy cost or the like.
As representatives of the wet methods, there may be listed those disclosed in J.P. KOKAI Nos. Hei 7-207307 and Hei 7-278619. The former discloses a method which comprises the steps of mixing an aqueous solution containing hydroxyl ions and ammonium ions with an aqueous solution of a water-soluble nickel (II) salt to form an ammonia-nickel complex and then adding a reducing agent to the ammonia-nickel complex to thus reduce the complex. On the other hand, the latter discloses a method which comprises the steps of adding a strong alkali to a nickel salt aqueous solution having a specific concentration, adjusting the temperature and pH of the mixture to specific values, treating it with a reducing agent having specific temperature and concentration and finishing the reaction within a specific reaction time. These patents disclose, as to the resulting nickel powder, that the primary particle size ranges from 0.3 to 1.2 .mu.m for the former and 0.4 to 0.6 .mu.m for the latter and that the widths of the particle size distribution thereof are identical or superior to those observed for the conventional products.
The powder prepared by the foregoing methods have a particle size falling within a certain range of the particle size distribution, but the powder prepared by the method disclosed in J.P. KOKAI No. Hei 7-207307 has a D.sub.90 value ranging from about 2.13 to 3.88 .mu.m as described in Table 2 on page 4 of the specification and that prepared by the method disclosed in J.P. KOKAI No. Hei 7-278619 has a D.sub.90 value ranging from about 2.58 to 2.87 .mu.m as described in Table 2 on page 3 of the specification. This clearly indicates that the foregoing methods are insufficient for preparing a powdery product which has a lesser extent of agglomeration, i.e., which has a small D.sub.90 value.